The invention relates to a power trench transistor.
Power trench transistors serve for switching high currents and must therefore have good breakdown properties (“avalanche strength”). The breakdown properties of a power trench transistor are greatly dependent on the configurations of the edge region surrounding the cell array (the cell array generally comprises a plurality of transistor cells connected in parallel). There are usually formed within the edge region edge constructions which primarily serve for setting electric field strengths between the cell array and a sawing edge of the power trench transistor to specific values. In order to avoid a premature breakdown in the off state of the power trench transistor, the electric field strengths within the edge region are not permitted to exceed specific maximum field strength values. For this purpose, it is attempted to configure the power trench transistor (the edge constructions) such that, in the off state, corresponding equipotential lines are led from the interior of the edge region to the surface of the power trench transistor in such a way that excessively great potential gradients are avoided. The defined leading-outwards is referred to as “equipotential line management”. Thus, by way of example, great curvatures and high equipotential line densities should be avoided since equipotential line profiles of this type may readily lead to undesirable breakdowns.
Premature breakdowns may occur in particular when power trench transistors are provided with additional drift zones (also referred to hereinafter as “epitaxial layer” or “pedestal epitaxial layer”) in order to increase their maximum reverse voltage.
FIG. 1 shows a detail from a cell array of a field plate trench transistor 1 with an additional semiconductor layer. The field plate trench transistor 1 has a front side contact 2, a rear side contact 3 and also a semiconductor body 4 arranged between front side contact 2 and rear side contact 3. There are formed in the semiconductor body 4 an n+-doped rear side connection zone 5, which usually serves as a carrier device, n+-doped source regions 6, p-doped body regions 7, n-doped drift regions 8 and also an additional n-doped drift zone 9 (generally an epitaxial layer), the doping of which is lower than the doping of the drift regions 8. Furthermore, a trench 10 is formed in the upper region of the semiconductor body 4, a gate electrode 11 being provided in said trench. The gate electrode 11 is electrically insulated from the semiconductor body 4 by an insulation layer 12, which is configured in thickened fashion (field oxide layer) in the lower region. The gate electrode 11 is furthermore insulated from the front side contact 2 by an insulation layer 13. The gate electrode 11 simultaneously serves as a field electrode (lower part of the gate electrode 11) and depletes the highly doped drift regions 8 in the off-state case. Without the field electrode, not all the charges present in the drift regions 8 could be depleted in the off state of the field plate trench transistor 1, whereby the blocking capability of the field plate trench transistor 1 would be greatly restricted.
If the additional drift zone 9 is omitted, then a ring trench which encloses the cell array of the trench transistor 1 and is filled or lined with thick oxide generally suffices to guarantee a sufficient breakdown strength in the edge region of the trench transistor. In this case, it is sufficient if the dimensions (depth/width) of the ring trench correspond to the dimensions of the cell array trenches, that is to say that the width/depth of the ring trench must not be higher than the widths/depths of the cell array trenches.
The situation is different if, as shown in FIG. 1, an additional drift zone 9 is provided between the drift regions 8 and the rear side connection zone 5. In order to be able to ensure a satisfactory breakdown strength in this case, too, it is known to increase the width or depth of the ring trench, or to provide a plurality of ring trenches. What is disadvantageous in this case is that the formation of a widened or deepened ring trench is complicated from a process engineering standpoint.
It would be advantageous to specify an edge construction for a power trench transistor, in particular for a field plate trench transistor, which, even when an additional drift zone is present, guarantees a sufficient breakdown strength and, at the same time, can be produced in a simple manner in terms of process engineering.